1. Field of the Invention
This invention relates to high-frequency resonant DC-to-DC power converters, and more specifically to such a converter in which output voltage regulation is maintained by adaptively varying the on time of two switching devices controlled by asymmetric pulse width modulation.
2. Description of the Prior Art
The evolution of present-day DC-DC converter technology focuses primarily on improvements in power density, efficiency, electromagnetic interference reduction, and thermal management. Silicon semiconductor devices are preferred as the main power elements because they are relatively inexpensive, are readily available, and come in many forms suitable for a large variety of designs. There is, however, a practical performance limit for conventional soft-switched topologies using such devices as the switching frequency is increased to improve power density. Consequently, full resonant topologies have become a desirable substitute for conventional modulator-controlled pulse width modulation (PWM) topologies.
One conventional type of resonant converter is the Class-D converter. Class-D converters have some unique advantages: 1) the maximum switching voltage across the switching devices is clamped to the input voltage, so when metal oxide silicon field effect transistors (MOSFETs) are used as the switching elements, devices with lower conduction resistance and lower withstanding voltage can usually be used; 2) when MOSFETs are used, the body diode is topology-friendly; 3) the maximum voltage across the transformer body is lower, thereby facilitating the transformer insulation design; 4) the waveform is partially sinusoidal, which results in lower harmonic content; 5) the resonant inductance can be integrated with the transformer; and 6) no large-value secondary choke is required for energy storage.
Class-D converters are typically operated on a 50--50 duty cycle in which each switching transistor is driven for slightly less than 50% of the operating frequency cycle, leaving short dead times in each cycle during which soft switching (i.e. switching at zero voltage and/or zero current to minimize dynamic losses) can occur. A Class-D converter operating in this mode at a fixed frequency is incapable of output regulation. Consequently, regulation in these converters is usually achieved by varying the switching frequency, a method which is not usable in synchronized circuits such as are common in the telecommunications industry.
U.S. Pat. No. 5,159,541 to Jain discloses a resonant converter which regulates output by varying the respective duty cycle of two switching transistors while keeping the frequency constant. This method of asymmetrical PWM of a resonant converter achieves output control by generating a variable-amplitude AC fundamental component in the power transformer input.
There are, however, two problems with this approach. One is that the topology of U.S. Pat. No. 5,159,541 requires a large capacitor in parallel with the MOSFET switch through which the resonant circuit discharges, in order to maintain lossless zero voltage switching of both switches. Large capacitors are an impediment to miniaturization and are therefore undesirable. If this capacitor is omitted, however, the MOSFET which charges the resonant circuit will not switch at zero voltage when the duty cycles of the two switches are made highly unequal in order to regulate the converter's output in the face of large input voltage variations. Such non-zero-voltage switching causes large power dissipation losses.
Another problem with variable duty cycle control is that the regulating asymmetry of the gate drive, which becomes more pronounced as the input line voltage increases, reflects to the secondary side of the isolation transformer and causes a very significant current magnitude difference between the rectifiers on the two secondary windings. This in turn causes a huge power dissipation on one of the rectifier arms, and consequently a large drop in efficiency at high line voltage (if low line is chosen as the 50% duty cycle condition).